Borussia Dortmund formally apologised yesterday to 1899 Hoffenheim for the unruly behaviour of their fans during a Bundesliga match on Sunday in Mannheim.
Dortmund fans chanted obscenities against the club during the match, which promoted Hoffenheim won 4-1. In particular they insulted club patron Dietmar Hopp, a local millionaire and co-founder of Europe's biggest software company SAP.
"What happened in Mannheim is embarrassing, shameful and dishonourable," said Dortmund club executive Hans-Joachim Watzke in a statement.
"We're going to do everything we can to prevent these sorts of people from damaging Dortmund's reputation," he added.
Mannheim police confirmed that they started an investigation after one man unfurled a banner explicitly encouraging violence against Hopp, whose heavy investment in the club helped them rise from the third to the first division in two seasons.
Dortmund said they will support the police investigation.
It was not the first time that opposing fans shouted abusive language at Hoffenheim and Hopp. Last month, Borussia Moenchengladbach club president Rolf Koenigs apologised after fans chanted obscenities against Hopp.